(a) Field of the Invention
This invention relates to incremental photoelectric measuring apparatus using either the Moire or the vernier fringe concept of measurement.
(B) Description of the Prior Art
The main element in an incremental photoelectric measuring system is an accurately ruled grating or scale of the desired length. This scale is commonly of glass or steel and has a precisely known number of lines per unit of length (for example, 500 equal lines and spaces per inch).
In a Moire fringe type system a short length of scale, commonly known as the index grating, having the same line structure as that provided on the scale is superimposed on the scale at a slight angle thereto such that the respective line structures cross. This produces a pattern of dark and light bands perpendicular to the lines on the scale as the lines on the index grating fall on lines or spaces of the scale. This integrated image is what is known as a Moire fringe pattern. The spacing of the Moire fringes can be adjusted by rotating the index grating relative to the scale about an axis which is perpendicular to both the scale and the index grating. In operation, when the index grating is translated along the scale, normal to the direction of the lines on the scale, the fringe pattern travels at right angles to the direction of movement. By passing a light beam through both the scale and the index grating, the movement of this fringe pattern may be converted into electrical signals by the use of photo-electric sensors. The output from such photo-electric sensors can be, for example, summed, shaped and amplified and then fed to an electronic digital display counter. The signals from the photo-electric sensors may also be used to give an indication of the direction of motion.
In vernier fringe type systems, the pattern of lines and spaces provided on the index grating is not the same as that provided on the scale. In one type of vernier system, the index grating has either slightly fewer or slightly more lines per inch than the scale. For example, a scale with 500 lines per inch could have an index grating with either 497 or 503 lines per inch. In this arrangement the index grating is superimposed over the scale such that the lines on the index grating are parallel to the lines on the scale. This produces a pattern of dark and light bands parallel to the lines on the scale as the lines on the index grating fall on the lines or spaces of the scale. This emigrated image is what is known as a vernier fringe pattern. The spacing of the vernier fringes is determined by the difference in pitch between the scale and the index grating. For example, a 500 lines per inch scale and a 503 lines per inch index grating generates 3 vernier fringes per inch. In operation, when the index grating is translated along the scale, normal to the direction of the lines on the scale, the fringe pattern travels parallel to the direction of movement. By passing a light beam through both the scale and the index grating, the movement of the fringe pattern may, by the use of photo-electric sensors etc., be converted into electrical signals to indicate both the amount and direction of movement.
A system using the Moire fringe concept of measurement is described in U.S. Pat. No. 3,833,303 to Richard H. Burns et al. The apparatus disclosed therein includes a scale, a transducer or reading head and a transducer support housing. The reading head includes an index grating supported on a carriage. The carriage includes a first set of scale engaging surfaces or pads which are designed to ride on the face of the scale and a second set of scale engaging surfaces or pads which are designed to ride on the edge of the scale which is ground and polished. These respective sets of pads are simultaneously held in engagement with the scale face and scale edge by a cantilever spring which is secured to the transducer support housing and connected to the carriage by a ball-and-socket arrangement. The scale is supported in a housing securable to one of two relatively movable members. The transducer support housing, which is secured to the other of the two relatively movable members, is positioned relative to the scale housing by a temporary alignment bracket.
The above described instrument is accurate to 0.0005 inches. However, it has been found that the arrangement of parts prevents its use when accuracy of 0.0001 inches is required. The limitation in accuracy is due, at least in part, to lack of conformity of the scale to the surface against which the housing is mounted. It has also been determined that for 0.0001 inch accuracy the end of the cantilever spring element fixed to the transducer support housing has to be more accurately positioned relative to the scale than the arrangement disclosed in U.S. Pat. No. 3,833,303 permitted.
To overcome the above described drawbacks, the present invention provides scale housing designed to insure that the scale is held in close conformity with the surface against which the scale housing is mounted. Further, the present invention insures that the transducer support housing and, hence, the mount for the cantilever spring element is carefully positioned relative to the scale.